Inertial-measurement systems are commonly used in determining the location and/or attitude of an object and in navigation. Such systems are particularly important when communication-based location determination and navigation approaches, e.g., global positioning system (GPS)-based or cell-phone-based approaches, are unavailable or undesirable. Among various inertial-measurement systems, strap-down MEMS-based systems are of significant interest due to their small size, low weight, low cost, and/or low power consumption.
A typical MEMS inertial-measurement system includes a MEMS inertial sensor, such as, for example, an accelerometer for sensing motion in a fixed direction or a gyroscope for sensing angular motion. The motion sensed by the sensor is typically translated into an electrical signal by sensor circuitry associated with the sensor. The sensed signal then represents the detected motion, such as an acceleration or a rate of rotation. One or more sensors and sensor signals may be combined to determine the location and/or attitude of an object to which the measurement system is strapped.
Generally, a “bias” (i.e., an error component) is present in the motion reading provided by inertial sensors, including MEMS sensors. The bias corresponds to an erroneous detection of motion by a sensor when the sensor is not actually moving. The bias of a sensor does not, however, always remain constant. For example, each time the sensor is turned off and then on, the bias may change—a change that is known as turn-on-to-turn-on bias (and is sometimes referred to herein as, simply, the “turn-on” bias). Moreover, as the sensor continues to operate, the bias can “drift,” i.e. change over time.
The bias, bias drift, or both can also change due to a change in temperature of the sensor. In a MEMS system, this phenomenon is often of great concern. In particular, to avoid the introduction of electrical noise into the sensor signal it is often desirable to locate the sensor circuitry in proximity to the sensor, e.g., within a few millimeters from the sensor. But, typically, when the sensor circuitry is turned on it heats up rapidly, which may cause the temperature of the sensor to also increase quickly due to the sensor's small size and proximity to the sensor circuitry. In addition, in some situations, when the object to which the inertial-measurement system is attached moves from one location at a certain temperature to another location at a substantially different temperature, e.g., from the inside of a building to the outside, the temperature of the inertial sensor may also change quickly. Such a change in the temperature of the sensor, whether caused by the environment or the sensor circuitry, can change the sensor bias and/or increase bias drift, causing the location, attitude, or navigation information obtained from the sensor to be erroneous.
One approach to mitigate or avoid these problems is to use calibration. For example, the temperature of the inertial sensor may be measured and the sensor reading as indicated by the sensor circuitry adjusted according to a temperature-sensitivity curve known a priori from extensive pre-deployment testing. Unfortunately, such calibration is generally not very effective at detecting turn-on-to-turn-on bias. It also fails to effectively nullify the effect of sudden, large changes in temperature on the bias that can occur when, for example, the environment of the sensor changes.
Another approach is to employ active thermal control. Under this approach, heaters, coolers, or both are typically used to maintain the sensor's temperature nearly constant, regardless of the change in temperature of the sensor circuitry or the sensor's environment. The use of heaters and/or coolers can, however, increase the cost, size, weight, and/or power consumption of the measurement system. For certain applications in which the inertial-measurement system should be small and should operate on limited power, active thermal management may be impractical or infeasible.
Needs therefore exist for improved systems and methods of MEMS based inertial measurement.